Myocardial Infarction, Angina & Heart Failure Flashcards
Myocardial Infarction (MI)
• Myocardial infarction also known as a heart
attack
• Death of heart muscle resulting from a
blockage of a coronary artery
• The blockage deprives the heart of oxygen
(ischemia) which causes injury to the heart
muscle, chest pain and may result in
irreversible death of heart muscle (infarct)
• Myocardial infarction are a result of a blockage of a coronary artery
• An infarction results in death of the affected myocardial cells
• Repair is by non-contractile scar tissue since myocardial cells are amitotic
• A major sign and symptom of an MI is pain
• Unlike angina this pain cannot be relieved by vasodilators and rest
Ischemic Heart Disease
• Ischemic heart disease is caused by an
imbalance between the myocardial blood flow and the metabolic demand of the myocardium.
• Reduction in coronary blood flow may be
related to progressive atherosclerosis with
increasing occlusion of coronary arteries.
• Blood flow can be further decreased by
superimposed events such as vasospasm,
thrombosis, or circulatory changes leading to
hypoperfusion.
Causes of atherosclerosis
• Atheroclerosis is a response to damage of the tunica intima • Damage may be caused by: – Bloodborne chemicals – Hypertension – Components of cigarette smoke – Viral or bacterial infections
atherosclerosis – Risk factors
• Risk factors include: Increasing age Smoking Kidney disease Being male Lack of exercise Obesity High blood cholesterol Diabetes Hypertension Stress Family history (genetics & environment)
Development of Atheromas
• Atheromas begin with damage to the endothelium (inner lining or tunica intima) of an artery
• Damage stimulates the immune system and inflammatory response to repair the injury
• Injured cells release chemotactic agents and mitosis inducing factors
• Macrophages attracted to the site, invade the damaged endothelium and take up lipids from the blood, particularly low density lipoproteins (LDLs – bad lipids)
• Lipids oxidise, further damaging cells and increasing chemotaxis. More macrophages arrive at the site
• Some macrophages transform to lipid-laden foam cells.
• The foam cells accumulate and form a fatty streak
• Neighbouring cells become damaged smooth muscle cells of the tunica media also take up cholesterol and begin to multiply
• Smooth muscles cells from the tunica media migrate to the damaged area and
deposit collagen and elastin fibres
• the tunica intima thickens with the formation of the an atherosclerotic plaque
• the wall can initially accommodate the plaque but eventually the plaque will grow
inwards into the lumen
• the plaque will continue to grow, calcium is deposited and the plaque becomes hard
• the constricted artery promotes platelet adhesion and thrombus formation
• constriction and increased rigidity promotes hypertension that in turn promotes increased deposition of lipids, increased hypertension etc
Coronary Atherosclerosis
• In general, the cross-sectional area of the
coronary artery lumen must be reduced by
more than 75% to significantly affect perfusion
• Coronary atherosclerosis is diffuse (involving more than one major arterial branch) but is often segmental, and typically involves the proximal 2 cm of arteries (epicardial).
• A 75% or more reduction will cause ischemia
Collateral Circulation
• Tiny collateral blood vessels grow when needed - both in number as well as in size.
• By the time a particular coronary blockage develops over a time, these multiple
collaterals grow and make alternative routes around the blockage creating a ‘Natural
Bypass’.
Angina Pectoris
- Blockage of the coronary artery may result in ischemia
- Ischemia may result in angina pectoris (‘choked chest’)
- Angina pectoris is a severe pain caused by a fleeting deficiency in blood delivery to the myocardium (lack of oxygen and increased lactic acid stimulate nerve endings)
- Myocardial cells are weakened but do not die
- Most commonly occurs when stressed or during physical activity when the oxygen requirements of the myocardium are not met
Treatment
• If the plaque is still ‘soft’ an angioplasty or stent may be inserted
- coronary bypass
-
Genetics of MI susceptibility
• Coronary heart disease is classed as a
complex genetic disease
• Many gene variants have an influence on
susceptibility and progression
• On-going research aimed at defining &
applying these genetic associations with CHD
Diagnosing Angina or Myocardial
Infarction
- Getting the history of pain
- Other signs and symptoms
- Taking vital signs (BP, P, Temp, BPM)
- Electrocardiogram
- Blood test for cardiac enzymes
Electrocardiogram (ECG)
- A 12 lead ECG is required to diagnose MI and which areas of the heart are involved. This makes it possible to anticipate complications
- A myocardial infarction may be indicated by a raised S-T segment in two or more lead tracings:
- Those with ST segment depression or T wave inversion (suspicious for ischemia)
- Confirmation of an MI may involve the presence of cardiac enzymes
Defibrillators
• Electric shock used to correct life-threatening heart rhythm
• Detects dangerous rhythm (eg. ventricular
fibrillation)
• Applies shock if necessary to stop heart
• Heart restarts
• Changed resuscitation rate from <5% to 70-80%
Cardiac Failure
• Myocardial infarction is a common cause of
cardiac failure
• Cardiac failure may involve the left and/or right ventricle(s)
• the term cardiac failure is often associated with a ‘heart attack’. However a heart can be failing whilst still beating
• Heart failure occurs when the heart fails to
pump sufficient blood to meet the body’s needs
• anything that resists cardiac output can result in cardiac failure
• cardiac failure can begin either on the right or left side. Can lead to failure of the other and complete failure
• when output is diminished, blood “backs up” within parts of the circulatory system
Left Ventricular Failure
• The left ventricle is the thickest muscle of the heart and needs the greatest blood supply. It is the first to suffer from
diminished blood supply
• Myocardial infarction is a common cause of cardiac failure, especially left ventricular failure
• Left ventricular failure causes back pressure on the lungs which may result in pulmonary oedema
• Left ventricular failure is a common cause of right ventricular failure
• When cells are damaged because of oxygen deprivation, potassium, calcium, and
magnesium leave the cells.
• Electrolyte disturbances cause decreased
contractility of the heart muscle
• Left ventricular failure causes back pressure, backward failure, on the lungs resulting in pulmonary oedema and congested lungs
